Various kinds of filters utilize nonwoven fabrics as filtration media. Nonwoven fabric materials are frequently used, for example, for automotive air filters, where it is important to remove airborne particles which may be suspended in the combustion air supplied to the automobile engine. In addition to their particulate holding abilities, filters beneficially allow high volumes of air to readily pass through. Consequently, filters are advantageously further characterized by high air permeabilities and low pressure drops. It has been heretofore been generally understood that improvements in particulate holding properties, i.e. the ability to trap solids, are typically made at the sacrifice of pressure drop and/or air permeability, i.e. the ability to allow gases to readily pass through.
Nevertheless, a number of effective air filtration products are known. For example, U.S. Pat. No. 5,397,632 to Murphy, Jr. et al., describes a nonwoven laminated composite article which is capable of undergoing pleating to form a stable pleated gas filtration medium. The pleating procedure provides the filtration medium with a three-dimensional area, thereby increasing its surface area. The filtration medium is a three layered composite nonwoven material which is needle punched to mechanically interconnect and interlock through entanglement of fibers and filaments the various webs forming the composite laminate material. These nonwoven layers include a fully bonded, air permeable, high loft polyester batt, an intermediate web of a substantially unbonded, air permeable continuous filament polyester web, and an outer backside web formed of random continuous filaments which are substantially fully bonded.
Another example of a composite laminate filter material used for particle removal from fluids is U.S. Pat. No. 5,283,106 to Seiler et al. This patent is directed to a multilayered filter specifically for dust collection from gases which includes the combination of two nonwovens to form a laminate material. The multilayer filter material can include a polyethylene terephthalate spunbonded web layer serving as a depth filter and a second fine filter joined to the depth filter by adhesive or welding. The depth filter has a basis weight of from 150 to 500 g/m2, and the filaments have a denier between 4 and 20 dtex. The fine filter has a basis weight of from 30 to 100 g/m2 and the filament denier between 1 and 6 dtex. The webs are subjected to a consolidating treatment such as calendering or needling or thermal bonding using binders.
Although a variety of pleated filters are known, a need remains in the art for filters providing improved particulate removal, particularly improved particulate removal values at higher air permeabilities and lower pressure drops.